As a fixing device used in an image forming apparatus such as a copying machine or a printer, there is a fixing device that generates heat in a conductive layer with an electromagnetic induction heating (IH) system and heats a fixing belt. In the fixing device of the IH system, in order to save energy consumption, for example, the heat capacity of the fixing belt is set small. There is a fixing device in which, if the heat capacity of the fixing belt is small, a magnetic shunt alloy is used to reduce variation of a heat value generated in the width direction of the fixing belt and compensate for a shortage of a heat quantity of the fixing belt.
In the fixing device including the magnetic shunt alloy, if the magnetic shunt alloy is heated to temperature close to the Curie temperature during high-speed printing, a magnetic characteristic of the magnetic shunt alloy is deteriorated. If the temperature of the fixing belt drops according to the deterioration in the magnetic characteristic of the magnetic shunt alloy, an IH driving circuit continues to feed a high-frequency current to an IH coil to raise the temperature of the fixing belt. If the IH driving circuit continues to feed the high-frequency current, a load applied to an element such as an insulated gate bipolar transistor (IGBT) of the driving circuit increases. As a result, it is likely that the element of the driving circuit is broken.
The related art is described in, for example, JP-A-2011-22446.